Process for the preparation of a pigment-fibre composite

ABSTRACT

The invention relates to a process for the preparation of a pigment-cellulosic fibre composite, wherein the pigment comprises an inorganic pigment which comprises local cationic charges, said process comprises contacting an aqueous cellulosic fibre suspension and inorganic pigment particles such that crystals of said pigment are attached to the surface of the fibre, said crystals having a size of at most 5 μm. The produced composite product can be used as a filler pigment or coating pigment in the production of paper.

TECHNICAL FIELD

The invention relates to a process for the preparation of apigment-cellulosic fibre composite product. The invention also relatesto the use of the pigment-cellulosic fibre composite product as acoating pigment or a filler pigment in the production of paper.

BACKGROUND OF THE INVENTION

A papermaking process starts with stock preparation where cellulosicfibres are mixed with water and mineral filler (usually clay or calciumcarbonate or also gypsum). The obtained slurry is delivered by means ofa head box on a forming fabric or press fabric or wire to form a fibrousweb of cellulosic fibres at the forming section of the paper machine.Then water is drained in the draining section and the formed web isconducted to the press section including a series of roll presses whereadditional water is removed. The web is then conducted to the dryingsection of the paper machine where most of the remaining water isevaporated typically by means of steam-heated dryer drums. Post dryingoperations include calendering where the dry paper product passesbetween rolls under pressure, thereby improving the surface smoothnessand gloss and making the caliper/thickness profile more uniform. Thereare various calenders such as machine calenders where the rolls usuallyare steel rolls and include a heated roll (thermo roll).

The mineral filler is usually introduced in the form of a dispersedfiller. Useful dispersing agents are the following: lignosulphonatessuch as Na lignosulphonate, condensation products of aromatic suplhonicacids with formaldehyde such as the condensed naphthalene sulphonates,dispersing anionic polymers, and copolymers made from anionic monomersor made anionic after polymerization, polymers containing repeatingunits having anionic charge such as carboxylic and sulphonic acids,their salts and combinations thereof. Also phosphates, non-ionic andcationic polymers, polysaccharides and surfactants may be used. Theamount of dispersing agent typically used is from 0.01 to 5.0%, such asfrom 0.05 to 3.0%, based on the weight of the mineral filler.

Gypsum or calcium sulphate dihydrate CaSO₄.2H₂O is suitable as materialfor both coating pigment and filler, especially in paper products.Especially good coating pigment and filler is obtained if the particulargypsum has high brightness, gloss and opacity. The gloss is high whenthe particles are sufficiently small, flat and broad (platy). Theopacity is high when the particles are refractive, small and of equalsize (narrow particle size distribution).

In the following the dimensions of gypsum particles will be discussed.However, what is said in respect of gypsum applies as well to otherpigments, such as calcium carbonate and kaolin.

The morphology of the gypsum product particles can be established byexamining scanning electron micrographs. Useful micrographs are obtainede.g. with a scanning electron microscope of the type Philips FEI XL 30FEG.

The size of the gypsum product particles is expressed as the weightaverage diameter D₅₀ of the particles contained therein. More precisely,D₅₀ is the diameter of the presumably round particle, smaller than whichparticles constitute 50% of the total particle weight. D₅₀ can bemeasured with an appropriate particle size analyzer, such as Sedigraph5100.

The flatness of a crystal means that it is thin. The form of flatcrystals is suitably expressed by means of the shape ratio SR. The SR isthe ratio of the crystal length (the longest measure) to the crystalthickness (the shortest transverse measure). By the SR of the claimedgypsum product is meant the average SR of its individual crystals.

The platyness of a crystal means that it is broad. Platyness is suitableexpressed by means of the aspect ratio AR. The AR is the ratio betweenthe crystal length (the longest measure) and the crystal width (thelongest transverse measure). By the AR of the claimed gypsum product ismeant the average AR of its individual crystals.

Both the SR and the AR of the gypsum product can be estimated byexamining its scanning electron micrographs. A suitable scanningelectron microscope is the above mentioned Philips FEI XL 30 FEG.

Equal crystal particle size means that the crystal particle sizedistribution is narrow. The width is expressed as the gravimetric weightdistribution WPSD and it is expressed as (D₇₅−D₂₅)/D₅₀ wherein D₇₅, D₂₅and D₅₀ are the diameters of the presumably round particles, smallerthan which particles constitute 75, 25 and 50%, respectively, of thetotal weight of the particles. The width of the particle distribution isobtained with a suitable particle size analyzer such as the abovementioned type Sedigraph 5100.

Gypsum occurs as a natural mineral or it is formed as a by-product ofchemical processes, e.g. as phosphogypsum or flue gas gypsum. In orderto refine the gypsum further by crystallising it into coating pigment orfiller, it must first be calcined into calcium sulphate hemihydrate(CaSO₄.1/2H₂O), after which it may be hydrated back by dissolving thehemihydrate in water and precipitating to give pure gypsum. Calciumsulphate may also occur in the form of anhydrite lacking crystallinewater (CaSO₄).

Depending on the calcination conditions of the gypsum raw material, thecalcium sulphate hemihydrate may occur in two forms; as α- andβ-hemihydrate. The β-form is obtained by heat-treating the gypsum rawmaterial at atmospheric pressure while the α-form is obtained bytreating the gypsum raw material at a steam pressure which is higherthan atmospheric pressure or by means of chemical wet calcination fromsalt or acid solutions at e.g. about 45° C.

WO 88/05423 discloses a process for the preparation of gypsum byhydrating calcium sulphate hemihydrate in an aqueous slurry thereof, thedry matter content of which is between 20 and 25% by weight. Gypsum isobtained, the largest measure of which is from 100 to 450 μm and thesecond largest measure of which is from 10 to 40 μm.

AU 620857 (EP 0334292 A1) discloses a process for the preparation ofgypsum from a slurry containing not more than 33.33% by weight of groundhemihydrate, thereby yielding needle-like crystals having an averagesize of between 2 and 200 μm and an aspect ratio between 5 and 50. Seepage 15, lines 5 to 11, and the examples of this document.

US 2004/0241082 describes a process for the preparation of smallneedle-like gypsum crystals (length from 5 to 35 μm, width from 1 to 5μm) from an aqueous slurry of hemihydrate having a dry matter content ofbetween 5 and 25% by weight. The idea in this US document is to reducethe water solubility of the gypsum by means of an additive in order toprevent the crystals from dissolving during paper manufacture.

U.S. Pat. No. 5,320,677 discloses a composite material produced bymixing gypsum and host particles, especially wood fibres, and sufficientwater to make a dilute slurry, followed by heating the slurry underpressure to convert the gypsum to calcium sulphate alpha hemihydrate,and then separating a major portion of the water, followed byrehydrating the hemihydrate back to gypsum. The composite material isuseful for making building and plaster products, especiallyfire-resistant wallboards.

DE 32 23 178 C1 discloses a process for producing organic fibres coatedwith one or more mineral substances. One embodiment comprises mixingcellulose fibres, calcined gypsum and water. The mixture is compacted togive a plastic mass which subsequently is dried and mechanicallycomminuted to give fine particles. The obtained product can be used asan additive or filler e.g. in bitumen masses or putties.

WO 2008/092990 discloses a gypsum product consisting of essentiallyintact crystals having a size from 0.1 to 2.0 μm. The crystalspreferably have a shape ratio SR of at least 2.0, more preferablybetween 2.0 and 50, and an aspect ratio AR between 1.0 and 10, morepreferably between 1.0 and below 5.0. The gypsum product can be used asa coating pigment or as a filler pigment in the production of paper.

WO 2008/092991 discloses a process for the preparation of a gypsumproduct wherein calcium sulphate hemihydrate and/or calcium sulphateanhydrite and water are contacted so that the calcium sulphatehemihydrate and/or calcium sulphate anhydrite and the water react witheach other and form a crystalline gypsum product. The formed reactionmixture has a dry matter content of between 34 and 84% by weight,preferably between 50 and 84% by weight. The gypsum product can be usedas a coating pigment or as a filler pigment in the production of paper.

The aim of the invention is to provide a pigment-cellulosic fibrecomposite wherein the pigment is attached fairly strongly to surface thefibre, which composite can be used as a coating pigment or a fillerpigment in the production of paper.

SUMMARY OF THE INVENTION

According to the present invention it was surprisingly found that afiller for use in the manufacturing of paper can be produced bycontacting an aqueous cellulosic fibre suspension and undispersed orpartially dispersed gypsum particles or some other undispersed orpartially dispersed pigment particles under such conditions that thepigment which comprises local cationic charges is attached to thesurface of the fibre to form a pigment-cellulosic fibre composite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows scanning electron microscope (SEM) micrograph of fibresfrom prior art gypsum filler in filler trial,

FIGS. 2-13 show SEM micrographs of gypsum/kraft pulp composites of thepresent invention at various initial pulp concentrations,

FIG. 14 shows SEM micrograph of gypsum/mechanical pulp composite of thepresent invention,

FIGS. 15-18 show SEM micrographs of PCC/kraft pulp composites of thepresent invention at various initial pulp concentrations,

FIG. 19 shows SEM micrograph of kaolin/kraft pulp composite of thepresent invention,

FIG. 20 shows SEM micrograph of gypsum+calcium sulfate hemihydrate/kraftpulp composite of the present invention, and

FIGS. 21-22 show SEM micrograph of titanium dioxide/kraft pulpcomposites of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect of the invention there is provided a process forthe preparation of a pigment-cellulosic fibre composite, wherein thepigment comprises an inorganic pigment which comprises local cationiccharges, said process comprises contacting an aqueous cellulosic fibresuspension and inorganic pigment particles such that crystals of saidpigment are attached to the surface of the fibre, said crystals having asize of at most 5 μm.

The pigment is attached to the fibre and consequently thepigment-cellulosic fibre composite is shown by most measurement methodsas a single piece. The shape and size of the pigment can roughly beestimated by means of microscopic images.

An important feature the present invention is that the pigment comprisesat least some sites that have a cationic charge. These cationicallycharged sites form together with the anionic fibres a bond between thepigment and fibres. The net charge of the pigment does not necessarilyhave to be cationic. The net charge can even be anionic.

In a preferred embodiment the net charge of the inorganic pigment isanionic.

Preferred inorganic pigment particles comprise gypsum, calciumcarbonate, especially precipitated calcium carbonate (PCC), kaolin,titanium dioxide, talc, silica or silicate. Especially preferred pigmentparticles are gypsum, calcium carbonate or kaolin. It is also possibleto use a mixture of two or more of these pigments. Preferably thepigments are introduced in the form of crystals and the pigments alsoappear as crystals attached to the fibres.

The inorganic pigment particles may additionally comprise calciumsulphate hemihydrate or calcium sulphate anhydrite which are convertedto gypsum crystals which are attached to the surface of the fibre, saidcrystals having a size of at most 5 μm. When the calcium sulphatehemihydrate or calcium sulphate anhydrite comes into contact with theaqueous cellulosic fibre suspension it will react with water to formgypsum crystals on the surface of the fibres. It is believed thatcalcium sulphate hemihydrate or calcium sulphate anhydrite acts as abinder and, thus, improves the adherence of the other pigment to thefibres.

In one embodiment gypsum crystals are attached to the fibre. The gypsumcrystals used in the production can have the shapes and sizes describedin WO 2008/092990 and WO 2008/092991. However, according to theinvention the gypsum crystals can also have other shapes, such as beingneedle-like.

The size of the pigment particles attached to the surface of thecellulosic fibre is preferably from 0.1 to 5.0 μm, more preferably from0.1 to 4.0 μm, and most preferably from 0.2 to 4.0 μm. The size of thepigment particles may also be from 0.1 to 2.0 μm or from 0.2 to 2.0 μm.

The pigment particles may be provided in the form of a powder or as aslurry, such as aqueous slurry.

Preferably the cellulosic fibre of the pigment-cellulosic fibrecomposite product comprises a chemical pulp, mechanical pulp includingchemimechanical pulp or deinked pulp fibre. Chemical pulps include kraftpulp and sulphite pulp. Mechanical pulps include stone groundwood pulp(SGW), refiner mechanical pulp (RMP), pressure groundwood (PGW),thermomechanical pulp (TMP), and also chemically treated high-yieldpulps such as chemithermomechanical pulp (CTMP). Deinked pulp can bemade using mixed office waste (MOW), newsprint (ONP), magazines (OMG)etc. Also mixtures of different pulps can be used. The net charge of thecellulosic fibre is preferably anionic.

Preferably the weight ratio of pigment to cellulosic fibre on dry basisin the produced composite is in the range from 70:30 to 10:90, morepreferably from 50:50 to 10:90, even more preferably from 40:60 to20:80, and most preferably from 40:60 to 30:70.

Preferably the cellulosic fibre content on dry basis in the contactingstage is from 0.1 to 15% by weight, more preferably from 0.2 to 10% byweight, and most preferably from preferably from 0.3 to 8% by weight.

Preferably the content of pigment on dry basis in the contacting stageis from 0.3 to 20% by weight, more preferably from 0.5 to 20% by weight,even more preferably from 1 to 15% by weight, and most preferably 2 to10% by weight.

Preferably the weight ratio of pigment to cellulosic fibre on dry basisin the contacting stage is from 95:5 to 20:80, more preferably from90:10 to 30:70.

Preferably the weigh ratio of pigment to water in the contacting stageis in the range from 0.005 to 0.6:1, more preferably from 0.01 to 0.6:1,and most preferably from 0.05 to 0.5:1.

According to the invention the cellulosic fibre may be unrefined orrefined cellulosic fibre. The refined cellulosic fibre preferably has alength of at most 5 mm.

The process of the invention may additionally comprise the step ofcomminuting the obtained product to form a comminuted pigment-cellulosicfibre composite product.

Furthermore, the process of the invention may additionally comprise thesteps of drying and comminuting the obtained product to form apigment-cellulosic fibre composite product in the form of particles.

According to the invention the pigment-cellulosic fibre compositeproduct may be produced in a pulp production unit and shipped as such orin a dewatered form or as a dry product to the paper manufacturing unitwhere the product, if necessary, can be diluted to desired consistency.Preferably the cellulosic fibre used for producing thepigment-cellulosic fibre composite product, is produced at this samepulp production unit.

The pigment may be provided in the form of an undispersed pigment or inthe form of a predispersed pigment.

According to the invention it is also possible to add a retentionchemical into the process for the preparation of the pigment-cellulosicfibre composite. Suitable retention chemicals include cationicpolyacrylamide and microfibrillated cellulose. Other suitable retentionchemicals are well known to the man skilled in the art.

According to the invention it is also possible to add a fixative intothe process for the preparation of the pigment-cellulosic fibrecomposite. Suitable fixatives are selected from the group consisting ofpoly aluminum chloride, poly diallyldimethylammonium chloride (polyDADMAC), anionic and cationic polyacrylates.

The process of the present invention can be carried out at atmosphericpressure and a temperature of between 0 and 100° C., preferably between0 and 80° C., more preferably between 10 and 50° C.

The process of the present invention is preferably carried out bymixing, preferably by mixing strongly, the aqueous cellulosic fibresuspension and pigment particles together for a sufficient period oftime, which can easily be determined experimentally. At high dry mattercontents strong mixing is necessary because, the slurry is thick and thereagents do not easily come into contact with each other. The initial pHis typically between 3.5 and 9.0, most preferably between 4.0 and 7.5.It is preferred that the initial pH is acidic, preferably between 3 and7, more preferably between 3 and 6. If necessary, the pH is regulated bymeans of an aqueous solution of NaOH and/or H₂SO₄, typically a 10%solution of NaOH and/or H₂SO₄.

Additional substances such as a natural or synthetic polymer binderand/or an optical brightener and/or a rheology modifier and/or sizingagents may be added to the produced pigment-cellulosic fibre compositeproduct. The sizing agent may be a rosin size or a reactive size such asalkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA).

The produced pigment-cellulosic fibre composite product can also betreated with other additives. A typical additive is a biocide whichprevents the activity of microorganisms when storing and using theproduct.

The present invention also relates to the use of a pigment-cellulosicfibre composite produced according to the above described process of thepresent invention, as a filler pigment or coating pigment in theproduction of paper.

According to the invention the pigment-cellulosic fibre composite may beadded to the short circulation or long circulation of a paper machine.

The “short circulation” refers to the system wherein water is separatedfrom the stock in web forming in the paper machine or drying sectionwire and used for dilution of the stock to be fed into the head box. Thewater separated from the stock is called “white water”.

The “long circulation” refers to the system in which excess white waterfrom the short circulation and other waters are collected at the papermachine and drying machine and used for stock dilution or other purposesin stock preparation.

The addition of the pigment into the pulp is typically carried out undermechanical share, especially by vigorous mixing. Vigorous mixing isneeded to provide sufficient collosion frequency and energy to obtaingood pigment/fiber interaction to increase pigment retention on thefibre as compared to the state of the art. Typical addition points aree.g. pulper, refiner, various continuous or batch type mixers, pumps orstatic mixers in e.g. pipes. Especially preferred mixers are high-speedmixers providing high shear forces, such as mixers of the Diaflo mixertype having a mixing speed of at least 500 rpm.

By using the pigment-cellulosic fibre composite in the production ofpaper, improved retention of the filler pigment and homogenous fillerdistribution can be obtained. This in turn makes it possible to uselower levels of retention agents and to obtain better printingbehaviour. Also improved strength and improved optical properties of thepaper can be obtained.

Additionally the present invention provides a paper product comprising apigment-cellulosic fibre composite produced according to the abovedescribed process of the present invention as a filler pigment orcoating pigment.

The paper product of the present invention preferably comprises inaddition to the pigment-cellulosic fibre composite, cellulosic fibres.

Preferably the cellulosic fibres comprise conventional papermaking pulpfibres including chemical, mechanical pulp including chemi-mechanicalpulp or deinked pulp fibres. Chemical pulps include kraft pulp andsulphite pulp. Mechanical pulps include stone groundwood pulp (SGW),refiner mechanical pulp (RMP), pressure groundwood (PGW),thermomechanical pulp (TMP), and also chemically treated high-yieldpulps such as chemithermomechanical pulp (CTMP). Deinked pulp can bemade using mixed office waste (MOW), newsprint (ONP), magazines (OMG)etc. Also mixtures of different pulps can be used.

Said cellulosic fibres can be similar to or different from the fibres inthe pigment cellulosic fibre composite product, and preferably thefibres are similar.

For fine papers the cellulosic fibres are preferably kraft pulp fibres.Examples of fine papers are writing and printing grade papers includingoffset, bond, duplicating and photocopying papers.

Preferably the amount of the pigment-cellulosic fibre composite in thepaper product is from 10 to 80%, preferably from 10 to 60%, morepreferably from 20 to 60%, and most preferably from 20 to 50% by weighton dry basis. Correspondingly the amount of said cellulosic fibres inthe paper product may be from 20 to 90%, preferably from 40 to 90%, morepreferably from 40 to 80%, and most preferably from 50 to 80% by weighton dry basis.

According to a preferred embodiment the pigment-cellulosic fibrecomposite constitutes the whole filler content of the paper product ofthe present invention meaning that no additional filler is added intothe paper manufacturing process. It is also possible that thepigment-cellulosic fibre composite forms only a part of the total fillercontent of the paper product, meaning that an additional filler, such asgypsum, PCC or kaolin is introduced into the paper manufacturingprocess. Preferably the pigment-cellulosic fibre composite forms atleast 50%, more preferably at least 60% by weight of the total fillercontent of the paper product.

According to one embodiment of the invention, the composite product is acoating pigment and comprises pigment crystals preferably having a sizeof between 0.1 and 1.0 μm, more preferably between 0.5 and 1.0 μm.According to another embodiment, it is a filler and comprises pigmentcrystals preferably having a size of between 1.0 and 5.0 μm, morepreferably between 1.0 and 4.0 μm. The pigment crystals in the fillercomposite may also have a size of between 1.0 and below 2.0 μm. Apreferred pigment crystal comprises gypsum crystal.

The gypsum crystals are preferably produced by a crystallizationprocess. Preferred gypsum crystals and processes for producing the sameare described in WO 2008/092990 and WO 2008/092991, the contents ofwhich are herewith included by reference. The gypsum crystal startingmaterial used in the present invention may be produced by the processesdisclosed in these documents.

According to WO 2008/092990 the gypsum crystals are produced bycontacting calcium sulphate hemihydrate or calcium sulphate anhydrite,water and a crystallization habit modifier under such conditions thatthe dry matter content of the reaction mixture is between 50 and 84% byweight.

According to WO 2008/092991 the gypsum crystals are produced bycontacting calcium sulphate hemihydrates or calcium sulphate anhydrite,water and optionally a crystallization habit modifier under suchconditions that the dry matter content of the reaction mixture isbetween 34 and 84% by weight.

In the processes for producing gypsum crystals, calcium sulphatehemihydrates, preferably β-calcium sulphate hemihydrate is typicallyused. It may be prepared by heating gypsum raw-material (such as naturalgypsum mineral or gypsum formed as a by-product of chemical processes,e.g. as phosphogypsum or flue gas gypsum) to a temperature of between140 and 300° C., preferably from 150 to 200° C. At lower temperatures,the gypsum raw-material is not sufficiently dehydrated and at highertemperatures it is over-dehydrated into anhydrite. Calcinated calciumsulphate hemihydrate usually contains impurities in the form of smallamounts of calcium sulphate dihydrate and/or calcium sulphate anhydrite.It is preferable to use β-calcium sulphate hemihydrate obtained by flashcalcination, e.g. by fluid bed calcination, whereby the gypsumraw-material is heated to the required temperature as fast as possible.However, it is also possible to use α-calcium sulphate hemihydrate inthe crystallization.

It is also possible to use calcium sulphate anhydrite as startingmaterial for producing gypsum crystals. The anhydrite is obtained bycalcination of gypsum raw material. There are three forms of anhydrite;the first one, the so called Anhydrite I, is unable to form gypsum byreaction with water like the insoluble Anhydrites II-u and II-E. Theother forms, the so called Anhydrite III, also known as solubleanhydrite has three forms: β-anhydrite III, β-anhydrite III′, andα-anhydrite III and Anhydrite II-s form pure gypsum upon contact withwater.

EXAMPLES

In the following the invention will be illustrated in more detail bymeans of examples. The purpose of the examples is not to restrict thescope of the claims. In this specification the percentages refer to % byweight unless otherwise specified and gypsum refers to calcium sulphatedihydrate.

The experiments were carried out at system pH with following equipments.

The reactor was of Hobart type N50CE or Diaf dissolver 20 VH (alsocalled Diaflo mixer). Mixing speed is about 250-500 rpm for the HobartN50CE mixer and about 500-1000 rpm for the Diaf dissolver 20 VH. Afterthe mixing the samples were disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles.

Analysis

Morphology of the pigments was studied by using FEI XL 30 FEG scanningelectron microscope. The pigment content of the samples was determinedas follows: When the mixing was stopped sample was disintegrated usingLorentz & Wettre wet disintegrator according to ISO 5263 standard using10000 cycles. Fiber samples were picked up and dried in Memmert agingoven; gypsum samples at 45° C., calcium carbonate and kaolin at 75° C.Ash content was determined for the dried samples using Nabertherm C250chamber furnace. Gypsum samples were heated to 850° C. and kept therefor three hours. Calcium carbonate and titanium dioxide samples wereheated to 525° C. for four hours and kaolin to 900° C. for two hours.Pigment content of the sample was weighted and for gypsum correctionfactor 1.265 was used to get the original calcium sulfate dihydratecontent from the calcium sulfate anhydrite ash. For other pigments nocorrection was used. The refined pine pulp had shopper-riegler value of29.

Zeta Potential Measurements

Zeta potential (Z, mV) was measured using Malvern Zetasizer ZS. Sampleswere prepared by taking 1 ml sample and diluted with 250 ml 1 mM KClsolution. pH was measured and adjusted either with dilute KOH or HCl.The results are set forth in following table.

Disp. Undisp. Kaolin 1 GCS Talc PCS Kaolin 2 Talc PCC Z, mV pH Z, mV pHZ, mV pH Z, mV pH Z, mV pH Z, mV pH Z, mV pH −18 3.5 −19 5.5 −43 3.8 −234.8 24 2.9 −19 3.2 −14 7.6 −21 4.3 −17 6.2 −63 7.3 −22 7.1 35 3.9 −273.9 −16 8.9 −66 5.5 −19 6.9 −68 8.1 −19 7.4 47 5.9 −48 6.3 −23 10 −556.1 −18 7.1 −64 9 −21 7.9 −58 6.2 −53 7 −75 8.4 −19 8 −20 8.9 −63 8.2−53 8.5 −76 9.3 −17 9 −62 8.9

Additionally, the measured zeta potential was −44 mV at pH 6.7 forrutile titanium dioxide, and −17 mV at pH 6.7 for anatase titaniumdioxide.

A negative zeta potential means anionicity.

GCS is ground gypsumPCS is precipitated gypsumPCC is precipitated calcium carbonateKaolin 1 is filler kaolinKaolin 2 is coating kaolin.

Reference Example 1

By contacting a fiber suspension and dispersed gypsum normally used asfiller in the production of paper, the gypsum particles are not adheredto the fibers to any notable extent. This is shown in FIG. 1 which showsscanning electron microscope (SEM) micrograph of fibers from prior artgypsum filler in filler trial. According to FIG. 1 only a few gypsumparticles are adhered to the fiber.

Example 1

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of eight percent and it was placed in the reactor. 91 gof undispersed gypsum filler pigment powder was added into 800 g of pinekraft pulp suspension under stirring.

Mixing was carried out for three minutes after the filler addition wascompleted. When the mixing was stopped sample was disintegrated usingLorentz & Wettre wet disintegrator according to ISO 5263 standard and10000 cycles. Fiber samples were picked up and scanning electronphotographs were taken. Pigment content of the disintegrated sample was25%. The obtained pigment-cellulosic fiber composite is shown in FIG. 2.

Example 2

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of eight percent and it was placed in the reactor. 45.5 gof undispersed gypsum filler pigment powder was added into 800 g of pinekraft pulp suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 18%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 3.

Example 3

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of six percent and it was placed in the reactor. 91 g ofundispersed gypsum filler pigment powder was added into 800 g of pinekraft pulp suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 34%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 4.

Example 4

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of four percent and it was placed in the reactor. 91 g ofundispersed gypsum filler pigment powder was added into 800 g of pinekraft pulp suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 34%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 5.

Example 5

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of two percent and it was placed in the reactor. 91 g ofundispersed gypsum filler pigment powder was added into 800 g of pinekraft pulp suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 36%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 6.

Example 6

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of one percent and it was placed in the reactor. 91 g ofundispersed gypsum filler pigment powder was added into 800 g of pinekraft pulp suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 30%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 7.

Example 7

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of eight percent and it was placed in the reactor. 91 gof undispersed gypsum filler pigment powder was added to small amount ofwater and mixed vigorously using Diaf dissolver 20 VH.

Pigment slurry was then added to 800 g of pine kraft pulp suspensionunder stirring. Mixing was carried out for three minutes after thefiller addition was completed. When the mixing was stopped sample wasdisintegrated using Lorentz & Wettre wet disintegrator according to ISO5263 standard and 10000 cycles. Fiber samples were picked up andscanning electron photographs were taken. Pigment content of thedisintegrated sample was 34%. The obtained pigment-cellulosic fibercomposite is shown in FIG. 8.

Example 8

Gypsum filler pigment was precipitated as described in WO 2008/092991.91 g of undispersed gypsum filler pigment powder was added to smallamount of water and mixed vigorously using Diaf dissolver 20 VH.Unrefined fiber suspension had solids content of eight percent and itwas placed in a beaker and mixed with Diaf dissolver as well. Pigmentslurry was then added to 800 g of pine kraft pulp suspension understirring. Mixing was carried out for three minutes after the filleraddition was completed. When the mixing was stopped sample wasdisintegrated using Lorentz & Wettre wet disintegrator according to ISO5263 standard and 10000 cycles. Fiber samples were picked up andscanning electron photographs were taken. Pigment content of thedisintegrated sample was 30%. The obtained pigment-cellulosic fibercomposite is shown in FIG. 9.

Example 9

Gypsum filler pigment was precipitated as described in WO 2008/092991.91 g of undispersed gypsum filler pigment powder was added to smallamount of water and mixed vigorously using Diaf dissolver 20 VH.Unrefined fiber suspension had solids content of four percent and it wasplaced in a beaker and mixed with Diaf dissolver as well. Pigment slurrywas then added to 800 g of refined pine kraft pulp suspension understirring. Mixing was carried out for three minutes after the filleraddition was completed. When the mixing was stopped sample wasdisintegrated using Lorentz & Wettre wet disintegrator according to ISO5263 standard and 10000 cycles. Fiber samples were picked up andscanning electron photographs were taken. Pigment content of thedisintegrated sample was 39%. The obtained pigment-cellulosic fibercomposite is shown in FIG. 10.

Example 10

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of four percent and it was placed in the reactor.Fennopol K3400R retention polymer (cationic polyacrylamide) was added tofiber suspension. 91 g of undispersed gypsum filler pigment powder wasadded into 800 g of pine kraft pulp suspension under stirring. Mixingwas carried out for three minutes after the filler addition wascompleted. When the mixing was stopped sample was disintegrated usingLorentz & Wettre wet disintegrator according to ISO 5263 standard and10000 cycles. Fiber samples were picked up and scanning electronphotographs were taken. Pigment content of the disintegrated sample was34%. The obtained pigment-cellulosic fiber composite is shown in FIG.11.

Example 11

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of four percent and it was placed in the reactor.Microfibrillated cellulose was added as retention aid to fibersuspension. 91 g of undispersed gypsum filler pigment powder was addedinto 800 g of pine kraft pulp suspension under stirring. Mixing wascarried out for three minutes after the filler addition was completed.When the mixing was stopped sample was disintegrated using Lorentz &Wettre wet disintegrator according to ISO 5263 standard and 10000cycles. Fiber samples were picked up and scanning electron photographswere taken. Pigment content of the disintegrated sample was 37%. Theobtained pigment cellulosic fiber composite is shown in FIG. 12.

Example 12

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of six percent and it was placed in the reactor. 73 g ofundispersed platy gypsum filler pigment powder was added into 800 g ofpine kraft pulp suspension under stirring. Mixing was carried out forthree minutes after the filler addition was completed. When the mixingwas stopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 26%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 13.

Example 13

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Groundwood pulp suspension hadsolids content of six percent and it was placed in the reactor. 90 g ofundispersed platy gypsum filler pigment powder was added into 800 g ofpine pulp suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 49%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 14.

Example 14

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CEmixer was used as reactor. Unrefined fiber suspension had solids contentof 6.5% and it was placed in the reactor. 91 g of undispersed aragonitecalcium carbonate filler pigment slurry was added into 732 g of pinekraft fiber suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 26%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 15.

Example 15

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CEmixer was used as reactor. Refined fiber suspension (SR (shopper rieglernumber)=29) had solids content of 4% and it was placed in the reactor.60 g of non-dispersed aragonite calcium carbonate filler pigment slurrywas added into 800 g of pine kraft fiber suspension under stirring.Mixing was carried out for three minutes after the filler addition wascompleted. When the mixing was stopped sample was disintegrated usingLorentz & Wettre wet disintegrator according to ISO 5263 standard and10000 cycles. Fiber samples were picked up and scanning electronphotographs were taken. Pigment content of the disintegrated sample was34%. The obtained pigment-cellulosic fiber composite is shown in FIG.16.

Example 16

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CEmixer was used as reactor. Refined fiber suspension had solids contentof 2% and it was placed in the reactor. 30 g of non-dispersed aragonitecalcium carbonate filler pigment slurry was added into 800 g of pinekraft fiber suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 40%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 17.

Example 17

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CEmixer was used as reactor. Refined fiber suspension had solids contentof 1% and it was placed in the reactor. 15 g of non-dispersed aragonitecalcium carbonate filler pigment slurry was added into 800 g of pinekraft fiber suspension under stirring. Mixing was carried out for threeminutes after the filler addition was completed. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 36%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 18.

Example 18

Hobart N50CE mixer was used as reactor. Unrefined fiber suspension hadsolids content of six percent and it was placed in the reactor. 100 g ofpredispersed platy kaolin filler pigment powder was added into 800 g ofpine kraft fiber suspension under stirring. Mixing was carried out forthree minutes after the filler addition was completed. When the mixingwas stopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Pigment content of the disintegrated sample was 26%. The obtainedpigment cellulosic fiber composite is shown in FIG. 19.

Example 19

Gypsum filler pigment was precipitated as described in WO 2008/092991.Hobart N50CE mixer was used as reactor. Refined pine kraft pulpsuspension had solids content of eight percent and it was placed in thereactor. 50 g of undispersed gypsum filler pigment powder and 50 gramsof calcium sulfate hemihydrate were added to the reactor. Pigment slurrywas then added to 800 g of pine pulp suspension under stirring. Mixingwas carried out for three minutes after the filler addition wascompleted. When the mixing was stopped sample was disintegrated usingLorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electronphotographs were taken. Pigment content of the disintegrated sample was34%. The obtained pigment-cellulosic fiber composite is shown in FIG.20.

Example 20

Precipitated Calcium carbonate filler pigment was taken. Threeexperiments with fiber solids of 0.7, 0.5 and 0.3% were carried out. 800grams of refined pine kraft fiber suspension (SR=29) was placed in abeaker and mixed with Diaf dissolver. 10.5, 7.5 and 4.5 g ofnon-dispersed aragonite calcium carbonate filler pigment slurry wasadded into 800 g of fiber suspension with the respective fiber solidsunder stirring. Mixing was carried out for three minutes after thefiller addition was completed. When the mixing was stopped sample wasdisintegrated using Lorentz & Wettre wet disintegrator according to ISO5263 standard and 10000 cycles. Pigment contents of the wetdisintegrated samples are shown in following table 1.

TABLE 1 Fiber Filler content of Filler content of the wet Filler solids(%) composite (%) disintegrated samples (%) retention (%) 0.7 66.0 44.667 0.5 66.4 38.0 57 0.3 67.6 33.0 49

Example 21

Retention of rutile form titanium dioxide filler pigment was tested.Refined fiber suspension had solids content of 4% and it was placed inHobart N50CE reactor.

30 g of rutile pigment slurry was added into 800 g of refined pine kraftfiber suspension (SR=29) under stirring. Mixing was carried out forthree minutes after the filler addition was completed and then mixed foran additional three minutes with Diaf dissolver. When the mixing wasstopped sample was disintegrated using Lorentz & Wettre wetdisintegrator according to ISO 5263 standard and 10000 cycles. Fibersamples were picked up and scanning electron photographs were taken.Ashing was carried out at 525° C. Pigment content of the disintegratedsample was 49.4%. The obtained pigment-cellulosic fiber composite isshown in FIG. 21.

Example 22

Retention of anatase form titanium dioxide filler pigment was tested.Refined fiber suspension had solids content of 4% and it was placed inHobart N50CE reactor. 30 g of anatase pigment slurry (calculated as drypigment) was added into 800 g of refined pine kraft fiber suspension(SR=29) under stirring. Mixing was carried out for three minutes afterthe filler addition was completed and then mixed for an additional threeminutes with Diaf dissolver. When the mixing was stopped sample wasdisintegrated using Lorentz & Wettre wet disintegrator according to ISO5263 standard and 10000 cycles. Fiber samples were picked up andscanning electron photographs were taken. Ashing was carried out at 525°C. Pigment content of the disintegrated sample was 50.7%. The obtainedpigment-cellulosic fiber composite is shown in FIG. 22.

1. A process for the preparation of a pigment-cellulosic fibrecomposite, wherein the pigment comprises an inorganic pigment whichcomprises local cationic charges, said process comprises contacting anaqueous cellulosic fibre suspension and inorganic pigment particles suchthat crystals of said pigment are attached to the surface of the fibre,said crystals having a size of at most 5 μm.
 2. The process according toclaim 1, wherein the inorganic pigment particles comprise gypsum,calcium carbonate, kaolin, titanium dioxide, talc, silica or silicate,preferably gypsum, calcium carbonate or kaolin.
 3. The process accordingto claim 2 wherein the inorganic pigment particles additionally comprisecalcium sulphate hemihydrate or calcium sulphate anhydrite which areconverted to gypsum crystals which are attached to the surface of thefibre, said crystals having a size of at most 5 μm.
 4. The processaccording to claim 1, wherein the net charge of the inorganic pigment isanionic.
 5. The process according to claim 1, wherein the net charge ofthe cellulosic fibre is anionic.
 6. The process according to claim 1,wherein the pigment crystals attached to the surface of the cellulosicfibre have a size between 0.1 and 5.0 μm.
 7. The process according toclaim 1, wherein the pigment particles are provided in the form of apowder or as a slurry.
 8. The process according to claim 1, wherein theaqueous cellulosic fibre suspension and the inorganic pigment particlesare contacted under high shear forces.
 9. The process according to claim1, wherein the weight ratio of pigment to cellulosic fibre on dry basisin the produced composite is in the range from 70:30 to 10:90,preferably from 50:50 to 10:90, more preferably from 40:60 to 20:80, andmost preferably from 40:60 to 30:70.
 10. The process according to claim1, wherein the cellulosic fibre content on dry basis in the contactingstage is from 0.1 to 15% by weight, preferably from 0.2 to 10% byweight, more preferably from 0.3 to 8% by weight.
 11. The processaccording to claim 1, wherein the content of pigment on dry basis in thecontacting stage is from 0.3 to 20% by weight, preferably from 0.5 to20% by weight, more preferably from 1 to 15% by weight, and mostpreferably from 2 to 10% by weight.
 12. The process according to claim1, wherein the weight ratio of pigment to cellulosic fibre on dry basisin the contacting stage is from 95:5 to 20:80, preferably from 90:10 to30:70.
 13. The process according to claim 1, wherein the weigh ratio ofpigment to water in the contacting stage is in the range from 0.005 to0.6:1, preferably from 0.01 to 0.6:1, more preferably from 0.05 to0.5:1.
 14. The process according to claim 1, wherein the cellulosicfibre comprises a chemical pulp fibre, such as a kraft pulp fibre, or amechanical pulp fibre including chemi-mechanical pulp fibre, or adeinked pulp fibre.
 15. The process according to claim 14, wherein thecellulosic fibre is refined cellulosic fibre, preferably having a lengthof at most 5 mm.
 16. The process according to claim 1, wherein thepigment-cellulosic fibre composite is prepared in a pulp productionunit.
 17. Use of a pigment-cellulosic fibre composite produced accordingto claim 1 as a filler pigment or coating pigment in the production ofpaper.
 18. The use according to claim 17, wherein the pigment-cellulosicfibre composite is added into the long circulation of the paper machine.19. A paper product comprising a pigment-cellulosic fibre compositeproduced according to claim 1 as a filler pigment or coating pigment.20. The paper product according to claim 19, wherein the amount of thepigment-cellulosic fibre composite is from 10 to 80%, preferably from 20to 60% by weight on dry basis.